Yuri Gagarin in the Vostok spacecraft on 12 April 1961. His orbit of the earth was beset by last-minute hitches. Photograph: RIA Novosti/TopFoto

His 108-minute journey into space made him the first man to orbit the Earth and one of the 20th century's greatest heroes, but Yuri Gagarin was almost undone by a wonky door and an overweight spacesuit.

Newly declassified documents show the Russian cosmonaut's flight into space 50 years ago this month was beset by last-minute technical hitches.

The formerly top secret Soviet papers are being released after a request to state archives by Russia's space agency, Roscosmos, to coincide with the anniversary next week of the world's first manned orbital flight, performed by Gagarin on 12 April 1961.

They show that shortly before takeoff a fault was discovered with the hatch of the cosmonaut's Vostok spacecraft. Engineers working at high speed were forced to loosen 32 screws and remove the hatch to mend a faulty sensor, which showed whether a hermetic seal had been achieved.

The archived documents – to be published on 12 April in Russian in a book, The First Manned Flight – also reveal that a day before the flight, scientists found that the combined weight of Gagarin, his spacesuit and his seat was 13.6kg (30lb) above the acceptable maximum.

In an attempt to reduce weight, engineers stripped away part of the Vostok's internal apparatus, but in their haste disconnected two gauges, one for pressure and one for temperature. In turn, that caused a short circuit, which specialists struggled to fix overnight.

Yuri Baturin, a space writer and former cosmonaut, previewed the files in the Novaya Gazeta newspaper on Wednesday. "Only now, from the declassified documents, do we find out what extraordinary situations and unexpected details accompanied man's first flight into the cosmos," he noted.

In a letter written to his family two days before the flight – to be opened in case he died – Gagarin, then 27, admitted that an accident could not be ruled out. "I believe completely in the technical equipment," he said, "but even on level ground a person sometimes falls over and breaks his neck."

The new documents flesh out tales of other defects and mistakes that have been reported since the flight, such as Gagarin's craft blasting to 203 miles instead of 143 miles because an engine failed to cut out. Also, the cosmonaut was unable to write in his logbook because his pencil floated off in zero gravity and he couldn't find it.

News of the cosmonaut's landing was beamed around the world and he quickly became a global celebrity, enjoying a rapturous welcome when he visited Britain in 1961. He died in mysterious circumstances in a plane crash in 1968.

Russia is preparing for a series of events to mark the anniversary of Gagarin's flight, including concerts, exhibitions and a meeting for the heads of about 40 foreign space agencies in the Kremlin. Charles Bolden, the administrator of Nasa, is due to attend.

Some intrigue was added to proceedings on Wednesday when Sergei Ivanov, deputy prime minister of Russia, announced abruptly that Anatoly Perminov, the head of Roscosmos, could be sacked this month.

"Anatoly Nikolayevich Perminov is 65," news agencies quoted Ivanov saying on a visit to Washington DC. "According to Russian law, no state official can work once he is over this age."

In practice, many officials serve longer than that, and the sacking would be a humiliation for Perminov. Rumours of his demise began in December after three satellites launched as part of the Russian Glonass navigation system crashed into the Pacific near Hawaii.

The delayed launch on Tuesday of a manned Soyuz capsule emblazoned with a portrait of Gagarin also provoked criticism of the Roscosmos boss. The Russian craft blasted off six days late to the International Space Station.

Perminov said in a statement that his possible ousting would not upset the Gagarin anniversary.

Cosmonaut honoured with Mall statue

Yuri Gagarin, the peasant farmworkers' son who became the most famous man on Earth when he went into space and orbited the planet 50 years ago, is to be celebrated with a statue on the Mall in London. The British Council announced that it is going to mark the achievements of the great Russian explorer by placing him opposite the statue of a great British explorer, Captain Cook.

Andrea Rose, the council's visual arts director and driving force behind the project, said the cosmonaut's successful mission in Vostok 1 was "a story that is of importance to all of us … the fragility and the daring and the bravery of the missions are something beyond recognition and are reasons why we wanted to celebrate Gagarin as a symbol of aspiration, as well as intellectual curiosity."

Rose said there was an imbalance in western knowledge of space history. We know the story of Apollo and Neil Armstrong but fewer of us now know the incredible story of Gagarin.

As well as the statue, the British Council will host an exhibition on his life which will include rare and intimate photographs loaned by Gagarin's family as well as artefacts such as an ejector seat and the first space suit, SK-1.

The project partly stemmed from Rose's professional connection with Gagarin's daughter, Elena Gagarina, director of the Kremlin museums where, next year, a Henry Moore exhibition will be held. Rose had been talking to Gagarina about lending Moore's double-edged sculpture – which has for the past 40 years been outside the Houses of Parliament – to Russia to display in the Kremlin gardens.

Rose then began thinking about what could be brought from Russia as a possible replacement for Moore's sculpture and so began the hunt for a suitable Gagarin statue.

The statue will be a copy of the one standing outside the cosmonaut's former school, Lyubertsy Vocational School No 10. The authorities there were reluctant to give up the original so the Russian space agency offered to have a cast made from the original moulds.

It will be installed on the Mall on 14 July for a year. The date and spot were chosen to mark the 50th anniversary of Gagarin's trip to London where he met the prime minister Harold Macmillan.

The statue will be made from zinc alloy and stand on a white Portland stone plinth. "We don't want kids swinging from the orbit," said Rose.

There are many more formal statues of Gagarin, but this jaunty one was praised by the cosmonaut's biographer, Piers Bizony. "It is a reflection of the man," he said. "Yuri Gagarin was charming, funny, sweet-natured and kind."

The statue is also a way of fostering good relations between the UK and Russia. Vitaly Davydov, state secretary and deputy head of the Russian space agency, said: "Gagarin belongs not only to Russia but to all countries and nations, and it's important to us that the statue of Yuri Gagarin will be shown in London – one of the world's most international and intercultural cities – to celebrate the 50th anniversary of the first manned spaceflight.

"Russia and the UK have much in common, not only as allies during the second world war and victory gained through sacrifice, but as nations which have always been eager to travel to the unknown and to discover new space. Gagarin symbolises this aspiration."